AutoREALM has default grid sizes that work well for a dungeon. You probably will want to turn on the Design Grid and Snap to Grid features (). You also may want to change the background color to black. Then use the Rectangle tool to layout your rooms and corridors on the graph lines (i.e. every other grid point). Door icons can be centered in between two grids (size them to the correct size after placement) in a hallway. Use the Rotate 90° button after you've placed a door to make vertically aligned doors.